[parser] Add robustness to data element token writing

[Enet4]

This tweaks the data element token writing routine so that instead of panicking from unexpected values in headers with the VR SQ, it warns about the case and gracefully tries to provide some tokens.

• A primitive value inside a data element with the VR SQ is not expected. I've added an edge case where the primitive data can be interpreted as raw DICOM bytes comprising the sequence items. This also means that it might produce malformed data inside those sequences, but this is a case of bad data in and bad data out.
• Pixel fragment sequences should only exist inside a pixel data header. I have chosen to skip the element entirely when this happens.

[qarmin]

I checked broken files from #626, and this PR fixes all problems that I had